Improvements for pwb.py:

- always add ".py" suffix if omitted.
  This enables running scripts outside the /scripts folder,
  e.g. library test functions as main module
- raise OSError when the given script name does not exist because
  "file not found" error is an OSError
  (in py3 which merged IOError to that)

Change-Id: Ie58e9573214accd013a8688816254173df1a5c33